1. Field of the Invention
The present invention relates to an integrated circuit that includes a non-volatile memory cell capable of temporarily holding information and that is suitable for application to a programmable logic device (PLD).
2. Description of the Prior Art
PLDs sometimes include a non-volatile memory such as a PROM (programmable ROM) capable of erasing/rewriting any data with ultraviolet rays or an electric signal, namely, an EPROM (erasable PROM), an EEPROM (electrically erasable PROM), and the like.
Memory cell 8 in FIG. 3 illustrates such a circuit including an EPROM in a PLD.
In the FIG., Tr.sub.1 is a floating gate transistor for non-volatilely holding written information, Tr.sub.2 is also a floating gate transistor sharing its gate with transistor Tr.sub.1, 10 is a word line for selecting a row into and from which any information is written and read out, 12 is a bit line for transferring an input write/read signal of information, 14 is a resistor for supplying supply voltage Vdd to transistor Tr.sub.1, 16 is a noise suppressing capacitor which is provided as needed, and 20 is an output line for transferring output programming data.
For writing information such as programming data into memory cell 8, a relatively high voltage is applied to word line 10 and bit line 12 to inject electric charges into the floating gate of transistor Tr.sub.2. Additionally, for reading information, a relatively low read voltage is applied to word line 10 to cause a change in current flowing through the previously written transistor, TR.sub.2, according to data written therein which transistor current change is in turn read through bit line 12 as the information stored in the transistor Tr.sub.1.
In an integrated circuit such as a PLD which includes a memory cell such as memory cell 8; however, it is needed, after writing various information in the memory cell to diagnose operation of the same upon inspection of the integrated circuit chip, to erase written information in the memory cell by irradiating the chip with ultraviolet rays or inputting an electric signal into the same for each such inspection. Such erasing is a time-consuming and wasteful operation, e.g., it takes 20 minutes for every inspection of an EPROM. This obstructs mass-production without finding any countermeasure to solve it.